1. Field of the Invention
The present invention relates to a developing device for developing an electrostatic image. More particularly, the present invention relates to a developing device for use in an electrophotographic image forming apparatus. In addition, the present invention also relates to an image forming apparatus and a process cartridge using the developing device.
2. Description of the Related Art
Recently, the development of office automation and colorization of documents have been remarkable. The need to print a document including figures (such as graphs made by a personal computer) by a printer and then copying the printed document to prepare materials for use in a presentation has been increasing, as well as the need to copy conventional full text documents. Because images produced by printers typically include solid images, line images and half tone images, printers are required to produce high quality solid, line and half tone images. In addition, printers are required to have high reliability.
In electrophotography, a developer is at once adhered to an electrostatic latent image formed on an image bearing member (e.g., a photoreceptor), and the resultant image is then transferred from the image bearing member onto a transfer medium such as a transfer paper, and finally fixed on the transfer paper. Specific examples of the developers configured to develop the electrostatic latent image formed on the image bearing member include two-component developers including a carrier and a toner and one-component developers consisting essentially of a toner (e.g., magnetic toner and non-magnetic toner). Two-component developers have the following drawbacks:
(1) Toner particles tend to adhere to the carrier, resulting in deterioration of charging property of the developer; and
(2) Because only the toner is consumed in a developing process, a device for controlling the toner concentration of the developer is needed, resulting in upsizing of a developing device.
One-component developers have advantages over two-component developers such that the size of an image forming apparatus can be minimized and an image forming apparatus using one-component developers can be used under various temperature conditions and humidity conditions. Therefore, one-component developers have been mainly used recently.
One-component developers are broadly classified into two categories: magnetic one-component developers consisting of a magnetic toner and non-magnetic one-component developers consisting of a non-magnetic toner.
In magnetic one-component developing methods, a magnetic toner including a magnetic material (such as ferrite) is held on a developing sleeve, containing a magnetic field generating mechanism (such as magnets) therein, generating a magnetic field by the magnetic force thereof, and a thin toner layer is formed by a toner layer thickness control member for developing electrostatic latent images. Magnetic one-component developing methods are broadly used recently in compact printers.
In contrast, in non-magnetic one-component developing methods, a non-magnetic toner is fed on a developing sleeve by pressing a toner supply roller thereto. Thereby, the toner is held on the developing sleeve by the electric force, and a thin toner layer is formed by a toner layer thickness control member for developing electrostatic latent images.
Non-magnetic one-component developing methods are broadly used because of having the following advantages:
(1) Non-magnetic toners can be preferably used for full color machines because of including no magnetic material which is typically colored (i.e., because toners having good color reproducibility can be provided); and
(2) The developing device can be miniaturized and the manufacturing cost thereof can be reduced because the developing sleeve includes no magnet.
However, one-component developing methods also have some drawbacks. In two-component developing methods, a carrier stably charges and transports a toner. In other words, a toner is transported to a developing sleeve to be developed after being sufficiently mixed with a carrier in a developing device. Therefore, the toner can be stably charged and transported even after a long repeated use. In addition, two-component developing methods can be easily applied to high-speed machines. In contrast, in one-component developing methods, there is no mechanism for stably charging and transporting a toner. Therefore, the toner cannot be stably charged and transported especially after a long repeated use, or when used in high-speed machines.
In particular, in non-magnetic one-component developing methods, a toner is transported to a developing sleeve and a thin toner layer is formed by a toner layer thickness control member, as mentioned above. In this case, a charging time in which the toner is contact-charged or friction-charged by frictional charging members (such as the developing sleeve and the toner layer thickness control member) is too short. Therefore, weakly or reversely charged toner particles are easily produced in one-component developing methods as compared with two-component developing methods.
As mentioned above, the toner is transported by the developing sleeve serving as a toner transport device to develop the electrostatic latent image formed on the image bearing member. Because the thickness of the toner held on the toner transport device is preferably as thin as possible, the toner is applied with a large pressure by the toner layer thickness control member. Therefore, external additive particles present on the surface of the toner particles are easily embedded therein, resulting in deterioration of chargeability and fluidity of the toner.
In attempting to solve these problems, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 08-122559 discloses an image forming apparatus including a magnet roller serving as a toner supply roller and a scraper serving as a toner layer thickness control member, both arranged in the vicinity of a toner supply opening formed between a toner supply tank and a toner hopper. The image forming apparatus further includes a toner supply roller driving device capable of rotating the toner supply roller in both forward and reverse directions. When the toner supply roller driving device rotates the toner supply roller, the following equation is satisfied:A<Bwherein A represents a rotation angle of the toner supply roller in the forward direction in a predetermined time, and B represents a rotation angle of the toner supply roller in the reverse direction in the predetermined time. It is described in JP-A 08-122559 that by using such an image forming apparatus, deterioration of fluidity of the toner in the hopper can be prevented, and the toner can be constantly supplied to the developing roller.
JP-A 2005-062215 discloses a developing device including a developing sleeve having a conductive resin layer thereon. The conductive resin layer has the same polarity as that of a developer, and includes at least a binder resin, a particulate conductive material, and a charge controlling agent. It is described therein that by using this developing device, the toner cannot be excessively charged on the developing sleeve having such a conductive resin layer. In addition, the rotation center of an agitation member configured to agitate the developer is arranged under the horizontal plane including the rotation center of the developing sleeve. It is described in JP-A 2005-062215 that with such a configuration, the agitation member does not supply an excessive amount of toner to the developing sleeve, resulting in quick charging of the toner.
However, these techniques are insufficient to stabilize chargeability and fluidity of non-magnetic one-component developers in the developing device.